This invention relates to a process for the separation and recovery of strontium values from nuclear wastes. More specifically, this invention relates to a process for the separation of recovery of strontium values from nuclear waste reprocessing solutions containing these values together with actinide and other fission product values.
The removal of .sup.90 Sr from acidic high-level liquid waste (HLLW) resulting from the processing of spent nuclear reactor fuels has long been recognized as a special problem. Strontium-90, together with .sup.137 Cs is one of the major generators of heat in nuclear waste. Thus, their presence complicates waste management options. The radioactive waste process stream from spent fuel reprocessing contains the transuranium elements (TRU), in addition to strontium and other components, in a solution with a high acid content on the order of 1 to 6 molar nitric acid. Because of the long-lived nature of the TRU elements, it is anticipated that they will have to be stored in geologically stable storage facilities for periods of time up to one million years. The strontium in the waste stream, if left in the waste as it is solidified for storage, will provide a significant amount of heat which must be removed from the stored solid waste. Thus, it is imperative that a satisfactory method be found for the removal of the strontium from the high-level liquid wastes before they can be solidified into a form suitable for long term storage.
Strontium-90, because of the heat which it generates, is also valuable as a reliable source of thermal energy for use in radioisotopic thermal electric generators.
A number of investigators have studied the separation of strontium from high level liquid wastes contained in strong acid solutions. Some work has focused on the use of a cobalt dicarbolyde as the extractant. The polyhedral dicarbolyde complexes of cobalt possess strong hydrophobic features which enable them to extract both Cs.sup.1+ and Sr.sup.2+ from nitric acid solutions in the concentration range of 0.01 to 0.1M (Sr) and up to 4-6M HNO.sub.3 (Cs) However, nitrobenzene, a highly toxic solvent, is the required diluent for this process. The presence of polyethylene glycols will also increase the distribution ratios of Sr.sup.2+, Cs.sup.1+ and trivalent metal ions, e.g. rare earths and transplutonium ions.
The macrocyclic polyethers ("crown ethers") such as dicyclohexano-18-Crown-6 (DCH18C6) have also been studied as strontium extractants. Many of these studies have utilized relatively volatile chlorinated hydrocarbons such as 1,1,2,2- tetrachloroethane, 1.2-dichloroethane and dichloromethane as diluents. These diluents are unsuitable for use in production facilities due to their toxicity and the potential for generating hydrochloric acid on radiolysis which is highly corrosive to process equipment. Furthermore, distribution ratios of strontium decrease rapidly when the nitric acid concentration of the feed solution exceeds about 2.0M.
Strontium extraction from acidic media by crown ethers is enhanced by using conventional acidic extractants such as bis(2-ethylhexyl)phosphoric acid (HDEHP) or dinonylnaphthalene sulfonic acid (DNNS). It has been shown that HDEHP and DNNS replace the relatively hydrophilic nitrate ion with a very lipophilic anion, significantly enhancing Sr recovery. To extract Sr from Purex raffinate, a four component process solvent formulation was developed consisting of 0.05M crown ether, 0.1M DNNS, and 25% tri-butyl phosphate (TBP) in a paraffinic hydrocarbon diluent. Although this 4-component system is the only process proposed for Sr extraction that is compatible with the PUREX process, effective back extraction of strontium from the extractant is highly inefficient making the process impractical.
The extraction of the nitrates of magnesium, calcium, strontium, and barium from nitric acid has also been tried using the crown ether, dicyclohexano-18- crown-6 (DCH18C6) in dichloroethane. Once again, as the concentration of HNO.sub.3 is increased, the distribution coefficient passes through a maximum at 1.5 to 2.0 mole/liter HNO.sub.3, with the extraction coefficients decreasing rather sharply with increasing concentrations of HNO.sub.3.
Still another study investigated the effect of other crown ethers on the extraction of strontium from a medium activity nuclear waste solution. The best distribution ratios were obtained with DCH18C6 in 1,1,2,2-tetrachloroethane from a solution containing 1.25 mol/l of HNO.sub.3, falling off rapidly at higher acid concentrations. Other diluents which gave similar results include chloroform, dichloromethane and nitrobenzene. While diluents such as chloroform, 1,2-dichloroethane, and 1,1,2,2-tetrachloroethane will elevate Sr distribution ratios to practical levels, these solvents are unacceptable in processing plants because of their toxicity and because of their generation of corrosive HCl on radiolysis.
All of the studies on the use of the various crown ethers in acceptable diluents show that the crown ethers are ineffective extractants for the recovery of strontium from nitric acid solutions. This is believed to be due to the need for the extractant to dehydrate the nitrate complex in order for it to transfer into the nonaqueous medium. In the case of Sr.sup.2+, however, the charge density of the dehydrated ion encircled by the cyclic polyether is probably insufficient to dehydrate the nitrate ion. Therefore, distribution ratios are quite low because of the energy required to transfer the hydrated anions into the organic phase. Since the acid concentration of most of the high-level waste solutions can range from 1 to 6 molar, the use of crown ethers is ineffective for the recovery of strontium from most of these solutions without considerable neutralization of the solutions, adding to the solution volume for which storage must ultimately be provided.
What is needed is an extraction system which can efficiently recover strontium values from aqueous solutions containing strontium and other fission product values and which contain up to about 6.0M in nitric acid and which is compatible with stainless steel and does not utilize a toxic solvent.